The present invention relates to a negative acting disc brake which is normally operative for braking under the action of a spring and is released by fluid pressure when necessary, and particularly to arrangements for automatically adjusting the clearance between the brake pads and the disc caused by abrasion of the brake pads.
A conventional disc brake of this type is disclosed in e.g. Japanese patent publication No. 51-43154 in which the disc brake comprises a piston opposed to a brake pad, an adjust bolt pressed against the piston and normally given such a turning torque by a spiral spring as to advance toward the piston, another piston threadedly engaging the adjust bolt and adapted to be retracted under fluid pressure to retract the adjust bolt when brake is released, said piston being retracted by means of a retractor provided in a cylinder.
In such a conventional disc brake, upon release of brake, the adjust bolt and the second piston move back to their original position and the piston is retracted by a predetermined stroke by the retractor. Therefore, as the brake pads abrade, some clearance will appear between the piston and the adjust bolt so that the adjust bolt will not press the piston any more. To prevent this, the adjust bolt is adapted to be turned in such a direction as to advance toward the piston by the force of the spiral spring to operatively press the piston.
In such a conventional disc brake, the clearance between the disc and the brake pad is automatically adjusted by advancing the piston for a distance corresponding to the amount of abrasion that occured at the last braking. In other words, the entire clearance therebetween is absorbed or adjusted by a single release of brake. Therefore, if the fluid pressure for brake release happens to be high, there is a possibility that the brake force after adjustment would be excessive.